An object is an instance of a class. It can be uniquely identified by its name and it defines a state which is represented by the values of its attributes at a particular time.
An object can be considered a "thing" that can perform a set of activities. The set of activities that the object performs defines the object's behavior.
The state of the object changes according to the methods which are applied to it. We refer to these possible sequences of state changes as the behavior of the object. So the behavior of an object is defined by the set of methods which can be applied on it.
Objects can communicate by passing messages to each other.